Physical state of tyrosine hydroxylase in the ganglia and nerve endings

Abstract

1. The distribution of tyrosine hydroxylase was studied in the cat superior cervical ganglia and in the nictitating membrane. The enzyme was present in a soluble and in a particulate form. 2. While in the cell bodies 76% of the total enzyme activity was found in the soluble fraction, in the nerve endings there was almost an equal proportion of both forms. 3. The K m for the substrate tyrosine was similar for both forms, but the K m for the cofactor, reduced pteridine, was 3 times lower for the particulate form than for the soluble form. 4. The concentration of dopamine required for 50% inhibition of tyrosine hydroxylase was similar for soluble and particulate forms of enzyme. 5. Heparin increased markedly the activity of the soluble form of the enzyme and had only a slight effect on the particulate enzyme. 6. Nerve stimulation at 5 or 10 Hz for 30 min did not result in a modification of proportions of soluble and particulate tyrosine hydroxylase, in spite of the fact that hydroxylation of tyrosine was increased under these conditions. 7. Phenoxybenzamine or reserpine did not increase the proportion of the particulate form.